1. Technical Field
The present invention is in the field of digital potentiometers.
2. Discussion of the Related Art
Digital potentiometers are electronic circuits that can provide a variable impedance as a result of processing a digital sequence. A digital potentiometer has a fixed-value impedance connected between two reference terminals. This impedance is provided by a string of impedance devices that can be selectively connected to a third terminal, called the wiper terminal herein, through electronic switches controlled by digital signals. Digital potentiometers can be used in digital to analog converters, and as replacements for mechanical potentiometers and rheostats.
The fineness of adjustment, resolution, or “granularity” of a digital potentiometer is typically determined by the number of digital bits used for the selection of the desired wiper position. For instance, an eight bit wiper address allows for 28 (i.e., 256) different wiper impedance selections, i.e., wiper positions. A disadvantage of finer adjustment granularity (more digital bits) is a rapid increase in the number of components (e.g., resistors, switches, decoding gates and other logic devices) required for implementation. The increase in the number of components typically results in larger and more expensive devices.
FIG. 1 is a schematic diagram of a digital potentiometer 100 that operates on the voltage-scaling principle. A resistor string including series-connected resistors R0, R1, . . . R2n−2 is connected between a high reference voltage (VREF+) terminal 102 and a low reference voltage (VREF−) terminal 104. The voltage drop across one of the resistors is equal to one least significant bit (LSB) of output voltage Vw change. The output analog signal Vw is collected on wiper terminal 106.
The wiper position is set by a switch decoding network, illustrated by decoder 108 and wiper switches S0, S1, . . . S2n−1. The wiper switches are typically implemented as one or more transistors. Decoder 108 receives input signals on line 110 and controls the operation of the wiper switches. The wiper switches tap different points in the resistor string, so that closing a particular wiper switch while leaving the other wiper switches open provides a unique ratio between the resistance values of the two resistor sub-chains connected to wiper terminal 106 via the closed wiper switch.
As mentioned, one disadvantage of this type of digital potentiometer is the relatively large number of components required. In particular, for n-bit resolution, digital potentiometer 100 requires 2n−1 resistors and 2n wiper switches. For example, in an eight bit implementation where digital potentiometer 100 includes 256 wiper positions, this approach would use 255 resistors and 256 wiper switches. In general, it is desirable to significantly reduce this large number of components for purposes of area savings, higher manufacturing yields, and lower costs. It is also desirable to reduce the number of transistor junctions connected at the terminals of each resistor element of the string of resistors. The parasitic electrical capacitance of each transistor junction used for the implementation of the analog switches between individual resistors adds to the distributed capacitance of the string of resistors and, thus, affects the AC frequency performance of the string of impedances. These transistors may also have leakage currents increasing with temperature, causing a temperature dependent variation of the current through the resistors greater than the variation of current expected due to the temperature coefficient of the resistors of the resistor string.